#!/usr/bin/perl -w

use Net::SNMP;
use strict;
use warnings;
use diagnostics;
use XML::Simple;
use Data::Dumper;

#open xml file
my $xmlfile = "e:/topo.xml";



my $ref = eval { XMLin($xmlfile) };


#erase or creat the ipfile
open ERASER, ">ipfile.txt";
close ERASE;

#see if open worked
if ($@)
{
    print "XML Read ERROR";
 } 
     else 
         {        
        #go to IPAddress tag and read infos into file
        foreach my $item (@{$ref->{Layer2Details}->{Device}}){
            
            my @ipliste = $item->{IPAddress};
                
            my @sorted = @ipliste;
            open OUTIP, ">>ipfile.txt";
            print OUTIP @sorted, "\n";
            close OUTIP;
        }
 }


 #open ipfile
 open IPFILE, "ipfile.txt" or die "Can't get IPs - $!\n";

    

                                                                      
#Sets knots und community          
my $community = 'public';
my $ifIndex   = '1.3.6.1.2.1.47.1.1.1.1.6';  
my $ifDescr   = '1.3.6.1.2.1.47.1.1.1.1.11'; 
my $ifDescr2   = '1.3.6.1.2.1.47.1.1.1.1.2';  


#erase or create files
open ERASER, ">serlist.txt";
close ERASE;
open ERASER, ">desclist.txt";
close ERASE;
open ERASER, ">finallist.txt";
close ERASE;



while ( my $ip = <IPFILE> )
{
    chomp $ip;
    print "Got: $ip\n";                                               
+                   

    #open session
    my ( $session, $error ) = Net::SNMP->session(
        -hostname  => $ip,
        -community => $community,
        -port      => 161
    );
    my $response;
                                                                      
    #goto index table and hash index
    if ( defined( $response = $session->get_table($ifIndex) ) )
    {
       
       
        #get serialnumbers
        foreach my $index ( values  %{$response} ) 
        {
        my $this_desc = "$ifDescr.$index";
            my $description;
            if ( defined( $description = $session->get_request($this_d
+esc) ) )
            {
           
        #print serialno. to file
            my @serial = values %{$description};
          open OUTPUT1, ">>serlist.txt";
          print OUTPUT1 @serial, "\n";
          close OUTPUT1;
    
        }
        }


    #get description
        foreach my $index ( values  %{$response} ) 
        {
        my $this_desc = "$ifDescr2.$index";
            my $description2;
            if ( defined( $description2 = $session->get_request($this_
+desc) ) )
            {


            #print describtion to file
            my @desc = values %{$description2};
            open OUTPUT2, ">>desclist.txt";
            print OUTPUT2 @desc, "\n";
            close OUTPUT2;
           
            }
               
       
    }   
    
    #creat final file
    open(OUT, "serlist.txt");
    open(OUT2, "desclist.txt");
    my @outlist;
    while (<OUT>)
        {
        my %hash;
        my @temp = split(/;/,$_);
        $hash{'file1'} = $temp[0];
        $hash{'file2'} = <OUT2>;
        
        #delete Return
        foreach (values %hash)
            {
            $_ =~ s/\n//g;
            }
        push(@outlist,\%hash);
        }
    close(OUT);

    #print to file
    open FINOUT, ">>finallist.txt";
    print FINOUT "$ip\n"; 
    foreach my $hashref (@outlist)
    {
        print FINOUT "$hashref->{'file1'};$hashref->{'file2'}\n";
    }
    print FINOUT "\n";
    close FINOUT;
    }
    

    #close session
    $session->close();
}
[download]

There is a commented version in the snippets and CUFP section

BTW you must put a -> between the Layer2Detail and the Device

Actually, the arrow is optional in that position. You need the first one, since without it, Perl would interpret $ref{Layer2Details} to mean the value associated with the 'Layer2Details' key in %ref, rather than the value associated with the 'Layer2Details' key in hash referred to by $ref.

All this and more is explained in Mark's very short tutorial about references which is part of your Perl distribution.